<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="/js/vue.js"></script>
</head>
<body>
  <div id="app">
      <p>{{msg}}</p>
  </div>
<br>
  <script>
   let app = new Vue({
   el:"#app",
   data:{ 
    km:0,
    m:0
      },
    methods: {
        kmtom:function(val){
            this.m=this.km*1000;
        },
        mtokom:function(val){
            this.km=this.m/1000;
        }
    }, 
    watch:{
        km:'kmtom',
        m:'mtokm'
    }
    })
  </script>
</body>
</html>